Constitutionalism refers to the entrenchment of secondary rules--i.e., rules that guide and constrain the adoption of ordinary (or primary) rules. To function effectively, a constitution's norms must be internalized by those who wield authority within the system and those who are subject to such authority. Moreover, the meaning and application of those norms must be relatively clear and remain relatively stable over time.
Yet constitutions, Americans especially must be reminded, are still human artifacts and mere instruments of governance. Like all human artifacts, they reflect the limits of the human mind, limits of foresight and of moral judgment. Like all instruments:
a. Design is important. Some constitutions perform better or worse at their desired function.
b. Performance is contingent. Each will perform best under a given set of circumstances and worse under other circumstances.
c. Endurance is not guaranteed. A perfectly suitable constitution may not age well. Significantly, not all required revisions can necessarily be accomplished within the procedures specified in the constitution itself.
As articulated masterfully by Rogers Smith, there are virtues and vices inherent in constitutionalism:
[T]he contributions of constitutionalism to good government on almost any conception of that term are many. If adhered to, constitutional procedures usually reduce the scope for arbitrary decision making by creating many motions that must be gone through before decisions are made and enforced by authorizing potential points of resistance among the many officials who must agree on a course of action. Constitutionalism, especially with a written constitution, can also help preserve whatever wisdom the original lawmakers may have had, which is considerable in the American case. And in many cases, constitutionalism also includes important substantive checks on governmental tyranny. Furthermore, by making rapid change more difficult, constitutionalism can promote stability, which makes it easier for people to plan their lives more securely. By establishing stable patterns of decision making that grow venerable with time, constitutionalism can also foster a sense of common memory and common cooperative endeavor among a political people that may promote both unity and belief in the system’s legitimacy. All these and others are advantages not to be foregone lightly.
Yet all those virtues have their attendant vices. Constitutional procedures may be unduly cumbersome in responding to national emergencies, notoriously so in wartime, but in other sorts of economic and social disasters as well. If a written constitution helps preserve the wisdom of its framers, so too it helps preserve their follies, their undue compromises of principle, their injustices. And in the very ways it contributes to stability, constitutionalism can help shelter unfair, exploitative status quo arrangements against necessary reforms. Insofar as it does so, it may provoke not gradual, orderly change with security but violent uprisings… Constitutionalism, then—like anything except perhaps the idea of the good itself—can be good or bad. Whether or not a decision conforms to it and preserves it is thus only part of the question of whether that decision is good or bad.
--Rogers Smith's "The Inherent Deceptiveness of Constitutional Discourse: A Diagnosis and Prescription" in Integrity and Conscience: Nomos XL (1998)
See Tom Ginsburg's comprehensive and engaging lecture on "Constitutions as Products."